Who is Lydia Glasher from Daniel Deronda and what is their importance?

Lydia Glasher is Grandcourt's mistress. The couple has four children. Lydia tells Gwendolyn that she is his mistress prior to Gwendolyn's engagement to Grandcourt. Gwendolyn consoles herself that, as Grandcourt's wife, she can be sure that Lydia and the children are taken care of. Lydia is angry at Grandcourt's marriage, but her son becomes the apparent heir to the baronetcy upon Grandcourt's death.